The experiments described in this proposal are directed at identification of the characteristics of water transport in the proximal convoluted tubule. First, we wish to determine if water flow from lumen to bath is predominately cellular (proceeds through cell membranes) or paracellular (proceeds through junctional complexes). We will use the isolated, perfused rabbit tubule technique. We hope to gain important insights into the interactions which water molecules experience in crossing the proximal tubular epithelium from comparisons between osmotic and diffusive water permeabilities and from the temperature dependence of the osmotic and diffusive water permeabilities. We will correct osmotic and diffusive water permeabilities for contributions of bulk-phase and intraepithelial unstirred layers and of cytoplasmic constraints to diffusion. Second, we wish to re-examine the reflection coefficients of the proximal convoluted tubule for NaCl, HaHCO3, and raffinose. Third, if the route of water flow from lumen to bath is transcellular, we wish to determine the reflection coefficients and osmotic water permeabilities of the peritubular cell membrane using the "crimped" isolated tubule technique, and of the apical and basal-lateral cell membranes using rabbit proximal tubule cell suspensions and high resolution, light-scattering techniques.